1. Field of the Invention
The present invention relates to an FM stereo demodulator for reproducing left and right signals, and more specifically to an FM stereo demodulator in which multiplication of an FM detection signal and a subcarrier signal is effected to reproduce the left and right signals.
2. Description of the Prior Art
FM stereo demodulators, for use in a stereophonic FM receiver for reproducing left and right signals, are divided into several types.
Among those prior art FM stereo demodulators, a switching type demodulator is popularly used.
In the switching type FM stereo demodulator, left and right signals are reproduced in a manner that a composite signal (FM detector output signal) is switched by a square-wave subcarrier signal of 38 kHz which is derived from a pilot signal contained in the composite signal.
Since the square-wave subcarrier signal includes frequency components of odd harmonics of the fundamental 38 kHz signal, such as 38.times.3=114 kHz, 38.times.5=190 kHz, there is a problem that a beat-frequency disturbance may occur between the odd harmonics of the subcarrier signal and a component of the intermediate frequency signal originating from a neighboring station.
In order to eliminate the beat-frequency disturbance, it is necessary to provide a so called anti-birdie filter (a kind of low pass filter) at an entrance of the stereo demodulation stage.
However, the use of low pass filters causes such problems as deterioration of the delay characteristic of the circuitry which extracts signal components from the composite signal and the reduction of amplitude of composite signal at higher frequency range thereof, which results in the deterioration of separation characteristics, and increase in the distortion of reproduced stereophonic signal.
Furthermore, signals passed through a low pass filter may involve distortions due to non-linearity of magnetic core material of the low pass filter.